Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 171500
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
800 AM PDT Tue Oct 17 2017

.DISCUSSION...17/12Z NAM in.

The northern hemispheric map is a bit amorphous this
morning...just a blob of cool air centered at the pole with a
hard-to-discern wave number...four maybe? However, this will
change later this week as the pattern transitions to a
progressive wave pattern. This will correspond with a transition
to an active weather pattern for the Pacific Northwest.

For now, flat ridging is in place over the west coast and a short
wave embedded in the flow will move onshore later this morning and
early this afternoon. This wave will push a weak front onshore. It
will bring some light precipitation to the north coast and lower
Umpqua Basin along with some gusty winds over the ridges and east
side this afternoon and evening. Overall the impacts will be
light.

The ridge, what there is of it, will break to the east of the CWA
Wednesday and southwest flow aloft will increase over the area as
a long wave trough deepens offshore. This will open the storm
door, and short waves ejecting from the offshore trough will
barrel through it.

The first few waves will be weak and the impacts will be similar
to the system moving onshore today. However, a much stronger one
will move onshore Thursday, and this will support a wet and windy
front which will also move onshore Thursday.

The mid shift summarized the impacts of this system well and they
are listed below:

*Impacts: Ponding on roadways, debris flows near burn scars near
 the coast and western Siskiyou County.
*Slippery, snow-covered roads over the higher passes in the
 Cascades near Crater Lake and Diamond Lake. Light snow on highway
 140 near Lake of the Woods.
*Travel may become difficult for high profile vehicles due to
 strong southerly winds along the coastal headlands, Shasta
 Valley, and east of the Cascades.
*Isolated trees may fall near burn scars due to high winds.

Snow levels will start off above 8000 feet on Thursday and will
drop to 4000 to 4500 feet by Friday evening as the precipitation
comes to an end. Some of the higher passes could see some moderate
snow, particularly near Crater Lake and Diamond Lake; but lower
passes like highway 140 near Lake of the Woods will only see light
snow. Siskiyou Summit may see a few flurries on Friday, but it is
more likely that the precipitation will end before it changes
over to snow.

Precipitation totals will be significant, especially at the
coast. Rain rates will be less than a quarter of an inch an hour,
but some coastal areas could see 1.5 inches in 6 hours and some
inland areas will get 1 inch in 6 hours. Rivers are still low and
soils are dry from the summer, but a few impacts could still be
felt, particularly for coastal rivers.

This will be one of the first wind events in our forecast area
for the season. The strongest winds will occur at the coast
Wednesday night into Thursday with Shasta Valley and East Side
winds ramping up Thursday into Friday morning. This could create
some travel difficulties, especially for high profile vehicles.
There may be some treefalls due to the winds, particularly near
recent burn scars.

The long wave upper trough will move onshore Friday, and short
waves will continue to move through the trough. This will support
widespread shower activity into Friday. The offshore upper level
ridge will amplify as it approaches the coast, and this will cause
the front to lift back north as a strong and quite wet warm front
Saturday, followed by a weaker trailing cold front Saturday night
into Sunday.

The offshore ridge will be quite strong by this time just off the
coast. Low level flow will turn easterly Sunday night into Monday,
so the area will dry out quickly at that time. Easterly flow will
weaken Monday into Tuesday, but it will remain dry with no
incoming systems initially. A weak front may bring some
precipitation to the north coast and Umpqua Basin Wednesday.

&&

.AVIATION...17/12Z TAF CYCLE...At the coast, fog and low ceilings
have set up IFR from Port Orford northward and extends in patches
into the Umpqua Basin including in the vicinity of KRBG. This
will remain until about 18Z today before lifting to VFR. That
being said, the stratus layer may remain near KRBG until the
afternoon. Have brought KRBG to VFR in the TAF because that seems
more likely. Elsewhere, VFR is expected to continue through the
next 24 hours. -Schaaf

&&

.MARINE...Updated 200 AM PDT Tuesday 17 October 2017...High
pressure to the southwest will maintain north winds through
tonight from Cape Blanco southward. Meanwhile, late this morning
an approaching front will brush the forecast area north of Cape
Blanco and introduce southerly winds there. Seas today will be
dominated by short period northwest swell. Winds will switch to
southerly Wednesday across the waters and increase ahead of a
strong cold front Wednesday night to Thursday morning, with gale
force winds possible mostly from Cape Blanco northward. Long
period and large northwest swells (around 20 feet at 17 seconds)
build into the waters late Thursday into Friday behind the front,
creating dangerous surf and bar conditions.

A warm front is expected to approach the coast late Friday night
into Saturday. Confidence is moderate to high that we could see
another round of gale force winds and hazardous seas. SK/FB

&&

.HYDROLOGY...The system arriving on Wednesday night into Friday
will provide a few hydrologic concerns. Although soils are dry and
rivers are low; there will be enough rain that ponding on roadways
could become a problem for areas of Curry, Coos, Josephine, and
western Siskiyou Counties. Additionally, debris flows near recent
burn scars will be possible. If you come across a flooded road,
find an alternate route. Remember to turn around, don`t drown.

Furthermore, for coastal rivers like the Chetco River and the
Pistol River, the recent burn scars have left quite a bit of
debris to float down the river with the heavy rains creating
dangerous bar conditions near the Port of Brookings. -Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Gale Watch from Wednesday evening through
        Thursday morning for PZZ350-356-370-376.

$$

15/15/08



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